NATURAL VARIABILITY IN AIR-SEA CO2 FLUXES DURING THE HOLOCENE AND THEIR EVOLUTION IN RESPONSE TO CLIMATE CHANGE.
Over the past decade (2013–2024), the ocean has absorbed approximately 2.9 ± 0.4 Gt C per year, or ˜ 30% of anthropogenic fossil fuel emissions. Changes in air-sea fluxes reflect the superposition of interannual to centennial variability in the natural carbon cycle and the response to antropogenic forcing. On interannual to decadal timescales, and thus far, natural climate variability modes appear to be the primary drivers of interannual fluctuations in the ocean carbon sink. Among these modes, ENSO (El Niño–Southern Oscillation) plays a dominant role. The year 2023 saw the combination of such an episode in the Equatorial Pacific and warm anomalies in extratropical and subpolar regions. The expected strengthening of the ocean carbon sink in response to the El Niño episode (decreased CO2 outgassing) was offset by excessive outgassing in extratropical and subpolar regions, particularly in the North Atlantic. This situation appears unprecedented and illustrates the interactions between the natural cycle and anthropogenic forcing (in this case, the warm ocean surface temperature anomaly). To understand contemporary and future fluctuations of air-sea CO2 fluxes, it is therefore important to understand their natural variability.
The Holocene is a period characterized by a relatively stable mean climate and patterns of climate variability similar to those observed today. This study takes advantage of this relative stability of the Holocene’s mean climate to investigate the natural variability of air-sea CO2 fluxes on interannual and decadal timescales. The aim is to establish a baseline against which to evaluate contemporary and future variability.
The first objective will be to confirm and quantify the role played by regional climate variability modes (e.g., ENSO, Equatorial Pacific) in the variability of air-sea CO2 fluxes during the Holocene. The next step will be to explore the existence of compensatory phenomena similar to those observed in 2023. Depending on the results obtained, the approach will subsequently be applied to historical and future periods.
Development of an autonomous module for glass alteration modeling and its coupling with reactive transport codes
In the context of the sustainable and safe use of nuclear energy within a carbon-free energy mix that addresses the climate emergency, managing radioactive waste inventory is a priority concern. The alteration of nuclear glass therefore directly affects the long-term assessment of the safety of geological storage of this waste. Understanding and simulating these processes is therefore a major scientific, industrial, and societal challenge. Existing models, such as GRAAL2 [1] developed at the CEA, capture the passivation mechanisms governing glass alteration, bridging nanometric processes to mesoscopic scale through mesoscopic-scale kinetic laws used in reactive transport codes (RTC).
This PhD aims to develop an autonomous glass module (GM) based on the GRAAL2 model, capable of computing glass alteration kinetics and interfacing with different reactive transport codes (HYTEC, CRUNCH…). The main objectives are: (i) to design and implement a kinetic module, (ii) to develop a coupling interface managing information exchange with RTC, (iii) to define and carry out numerical validation campaigns on reference test cases for both the GM and the coupler, and (iv) to perform sensitivity and uncertainty analyses to identify the key parameters controlling glass behavior in a multi-material context (glass, iron, clay).
The PhD will take place at the Laboratory for Environmental Transfer Modeling (LMTE), within the IRESNE Institute (CEA, Cadarache site, Saint-Paul-lès-Durance). The project will provide the PhD candidate with cross-disciplinary skills in geochemistry, multiphysics coupling, and scientific software development, opening career opportunities in both academic research and nuclear/environmental engineering.
References:
[1] M. Delcroix, P. Frugier, E. Geiger, C. Noiriel, The GRAAL2 glass alteration model: initial qualification on a simple chemical system, Npj Mater Degrad 9 (2025) 38. https://doi.org/10.1038/s41529-025-00589-4.
Development of a transport chemistry model for spent fuel in deep geological disposal under radiolysis of water
The direct storage of spent fuel (SF) represents a potential alternative to reprocessing as a means of managing nuclear waste. The direct storage of spent fuel in a deep geological environment presents a number of scientific challenges, primarily related to the necessity of developing a comprehensive understanding of the processes involved in the dissolution and release of radionuclides. The objective of this thesis is to develop a comprehensive scientific model that can accurately describe the intricate physico-chemical processes involved, such as the radiolysis of water and the interaction between irradiated fuel and its surrounding environment. The objective is to propose an accurate reactive transport model to enhance long-term predictions of storage performance. This thesis employs a back-and-forth process between modeling and experimentation, with the goal of refining the understanding of alteration mechanisms and validating hypotheses with experimental data. Based on existing models, such as the operational radiolytic model, the work will propose improvements to reduce the current simplifying assumptions. The candidate will contribute to major industrial and societal issues related to nuclear waste management and will help to provide solutions to the associated safety issues.